Fever of Unknown Origin in the Foal

"Take an organized approach to elucidate causes of fever of unknown origin in the foal," said Robert Franklin, DVM, Dipl. ACVIM, of Goulburn Valley Equine Hospital in Victoria, Australia, during the Western Veterinary Conference held February 15-19 in Las Vegas, Nev.

Normal foal body temperature is up to 102°F, he said, and it can be elevated by infectious and non-infectious factors. Infectious causes include bacteria (the "default" diagnosis), viruses (often rotavirus), and fungi. Non-infectious causes might be neonatal isoerythrolysis (NI, or jaundice), a reaction to a transfusion, hot weather, transport stress, exercise, certain medications (particularly macrolides, including erythromycin and azithromycin), hypothalamic problems, and muscular disorders such as white muscle disease (muscle damage and weakness caused by Vitamin E and/or selenium deficiency).

The Physical Examination"Don't rush this even if your day is behind schedule," Franklin said. "It doesn't save any time if you have to go back to look at the foal again." He recommended evaluating the following characteristics in the foal in addition to temperature:

Respiratory function. Problems can be occult (illness without obvious clinical signs), he said.

Diarrhea. This is often caused by rotavirus, Salmonella, and Clostridium.

Icterus--dark brownish-yellow urine and yellow sclera (what is normally the white part of the eye). This can indicate NI or jaundice.

Systemic shock (characterized by collapse or coma).

Lameness. This can be linked to a fever, although it might be difficult to clarify that link.

Swellings and abscesses. These might be due to injection site abscesses, bone infections, or infectious causes such as strangles.

Sepsis (systemic infection). This is characterized by multiple sites of infection, eye inflammation, pinpoint hemorrhages on the gums and vulva, scleral hemorrhages (of the tough white covering of the eyeball), and fever.

What to KnowAs an internal medicine specialist, Franklin relies on a variety of tests to identify what's going on inside the foal and quickly arrive at an appropriate diagnosis and treatment. He listed the following information as the minimum facts he requires on feverish foals:

Complete blood count (CBC) and fibrinogen levels. This gives a practitioner a picture of what's going on with the foal's immune response to bacteria, viruses, and fungi.

Biochemistry profiles might indicate organ dysfunction.

Levels of IgG, a type of immunoglobulin or immune system protein, which helps identify failure of passive transfer (lack of colostrum).

Rebreathing examination. This is when you cover the foal's nostrils briefly so you can assess lung sounds when he breathes deeply after you uncover his nostrils.

Blood culture. "When a foal comes in with suspicion of sepsis (systemic infection), if I could only do one test it would be a blood culture as it would identify the cause and directly affect treatment," Franklin said. "It doesn't cost much, and over 50% of septic foals will be positive. Human hospitals are great for testing these samples if you need them in a hurry and can't do them yourself."

Skeletal radiographs and arthrocentesis (joint taps to sample joint fluid). "This can help you identify bone problems such as growth plate infections, and you can perform joint fluid analysis and obtain culture results," he said. "It's better to use the right antibiotic based on culture results than to continually medicate the foal with a variety of antibiotics because you don't know what bacteria is."

Chest radiograph. "This can help you identify occult pneumonia very easily," he stated. "More than 90% of fevers come from the chest and abdomen."

Thoracic (chest) ultrasound. Used for imaging the lung surface. (See article #5090 for more on this.)

Transtracheal wash. This can identify pathogens present in the lungs and direct antibiotic selection. "There is no time to play musical antibiotics with these foals," he stated.

Abdominal ultrasound. This can help identify peritonitis, enteritis, abdominal abscesses, etc. (See article #5090 for more on this.)

Abdominocentesis. Used when fluid is seen on the ultrasound examination.

Fecal analysis. Looking for rotavirus and Salmonella in culture and Clostridium difficile toxin on an ELISA test.

Start Treatment Quickly!Even if you aren't yet sure of the foal's problem because the tests are still out or inconclusive, treatment is necessary if the patient appears compromised. Franklin said practitioners should initially assume a foal has a bacterial infection, as this is the most serious and treatable cause of a fever of unknown origin in the foal. Reasons to stop suspecting current bacterial infection include a lack of lab evidence, reduction of fever in response to a change in environment, or another logical explanation.

"Start with a broad-spectrum antibiotic and continue this treatment until the blood work comes back normal, fever goes down, and the foal begins to act normal again," he said. "Change your approach if the foal doesn't improve (in which case you would switch antibiotic classes, but stay broad-spectrum) or if culture results come back and indicate a change in medication.

"Believe your tests and give each antibiotic three days to work or you won't get anywhere," he emphasized.

"Don't just wait and hope it goes away, or randomly switch antibiotics," he urged the audience. "Do keep investigating until you solve the problem."